So long as you are logged in (http://www.JournalFen.net/login.bml), you will see this option on everyone's User Info page. It is a small icon with a "+" symbol, located in the blue toolbar at the top of the userinfo page. When you click on that icon, you will be prompted to confirm your decision before the user is added to your Friends list (see http://www.JournalFen.net/support/faqbrowse.bml?faqid=61 for information on managing your Friends list).

If the user is already listed as a Friend, this button allows you to modify settings (such as colors and custom Friends groups; see http://www.JournalFen.net/support/faqbrowse.bml?faqid=102 for information on custom Friends groups) or to remove them from your Friends list.